Prior to the present invention, as shown by Berger U.S. Pat. No. 4,395,527 and Lee U.S. Pat. No. 4,586,997, incorporated herein by reference, silicone polyimides were generally recognized as materials useful as protective coatings for semiconductors and other electronic devices. As discussed by Lee and Berger, silicone polyimides are insoluble in many of the common organic solvents. However, particular siloxane imide copolymers have been found to be soluble in glycol methyl ethers, such as diglyme, or mixtures of diglyme with organic solvents, such as xylene, which can serve as an azeotroping solvent.
Although silicone polyimides made in accordance with the teaching of Lee and Berger have been found useful as protective coatings for a variety of semiconductors, it has been found that the 8000 g/mole molecular weight range of silicone polyimides made in diglyme mixtures often does not provide the thermal stability required in particular microelectronic applications. Silicone polyimides are often needed having a higher molecular weight of at least 30,000 g/mole or more.
One method for increasing silicone polyimide molecular weight to provide higher thermal stability is to form the silicone polyimide in a chlorinated aromatic hydrocarbon solvent, such as orthodichlorobenzene, in the presence of an imidization catalyst, for example, p-N,N-dimethylaminopyridine. It has been found, however, that even though the resulting silicone polyimide has improved thermal stability, its solution viscosity as a result of its higher molecular weight, can be substantially enhanced. For example, the solution viscosity of a silicone-polyimide of 30,000 g/mole at 30% solids in diglyme/xylene can be as high as 8000 centipoise rendering it less useful as a spin castable mixture. In addition, the higher molecular weight silicone polyimide retains residual amounts of the imidization catalyst required in its polymerization, which can further reduce its utility as a coating material in microelectronic applications. Although a somewhat improved viscosity can be achieved by polymerizing and utilizing the silicone polyimide in a chlorinated hydrocarbon solvent, such as orthodichlorobenzene, the utility of such mixture is significantly reduced because of environmental considerations and its potential for corrosion as a result of the presence of chemically combined halogen.
The present invention is based on the discovery that certain silicone polyimides useful as coating materials in microelectronic applications can be made by effecting reaction in the presence of methylanisole, which hereinafter means o-, p-, or m-methylanisole and preferably p-methyanisole between particular aromatic dianhydrides and C.sub.(6-14) aryl diamine or a mixture thereof with aminoalkyl terminated polydiorganosiloxane. There can be used aromatic bis(ether anhydride), for example 2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride "BPADA", or phthalic anhydride terminated polydiorganosiloxane such as bis(phthalic anhydride)tetramethyldisiloxane "PADS". These aromatic dianhydrides can be reacted with C.sub.(6-14) aryldiamines, such as toluenediamine "TDA", or polydiorganosiloxane having terminal .alpha.-aminoalkyl diorganosiloxy units, such as tetramethyldisiloxane having terminal .alpha.-aminopropyl dimethylsiloxy units "DGAP", or where the methylsiloxane block has eight chemically combined dimethylsiloxy units, "D.sub.8 GAP".
Unexpectedly, these silicone polyimides made in the presence of methylanisole, exhibit a thermal stability which is substantially equivalent to the thermal stability shown by silicone polyimide made from the same ingredients and proportions in orthodichlorobenzene in the presence of an imidization catalyst. Further, silicone polyimide made in methylanisole, enjoys a significantly lower solution viscosity when employed at concentrations of 30% or more by weight of solids, as compared to the same silicone polyimide in an ortho-dichlorobenzene or diglyme media.